Avoid using list_length() to test for empty list.

The standard way to check for list emptiness is to compare the
List pointer to NIL; our list code goes out of its way to ensure
that that is the only representation of an empty list.  (An
acceptable alternative is a plain boolean test for non-null
pointer, but explicit mention of NIL is usually preferable.)

Various places didn't get that memo and expressed the condition
with list_length(), which might not be so bad except that there
were such a variety of ways to check it exactly: equal to zero,
less than or equal to zero, less than one, yadda yadda.  In the
name of code readability, let's standardize all those spellings
as "list == NIL" or "list != NIL".  (There's probably some
microscopic efficiency gain too, though few of these look to be
at all performance-critical.)

A very small number of cases were left as-is because they seemed
more consistent with other adjacent list_length tests that way.

Peter Smith, with bikeshedding from a number of us

Discussion: https://postgr.es/m/CAHut+PtQYe+ENX5KrONMfugf0q6NHg4hR5dAhqEXEc2eefFeig@mail.gmail.com

src/backend/utils/adt/selfuncs.c

@@ -3408,7 +3408,7 @@ estimate_num_groups_incremental(PlannerInfo *root, List *groupExprs,
 	 * for normal cases with GROUP BY or DISTINCT, but it is possible for
 	 * corner cases with set operations.)
 	 */
-	if (groupExprs == NIL || (pgset && list_length(*pgset) < 1))
+	if (groupExprs == NIL || (pgset && *pgset == NIL))
 		return 1.0;
 
 	/*